1. Field of the Invention
This invention relates to a method and apparatus for comminuting solid particles and more particularly to such a method and apparatus which utilizes pressurized gas and a stationary velocity head to drive the particles upwardly in a chamber through rotors and semipermeable screens which forms a vertical fluid bed, vortices, and cyclones which effect both the comminuting and classifying action.
2. Description of the Related Art
Micronized limestone is used in agriculture, industrial fillers, environmental controls and the construction trade. Micronized cement is useful in the building industry.
Micronized coal is used as an energy source in the generation of electricity and micronized limestone/dolomite or lime are used for environmental compliance in the flue gas cleanup of power plants. Micronized coal burns with a flame velocity similar to natural gas or fuel oil and with a short flame that allows the heat energy generated in the combustion to be readily transferred to the water walls of the boiler. This results in an increased boiler rating and less heat losses through the ducts and flue stack. In addition to providing a more complete combustion, the micronized coal upon combustion yields a micronized fly ash with low carbon content which is of considerable value in the construction industry as a substitute for cement in high strength concrete formulations.
In addition the combustion of micronized coal requires no excess air and results in minimized NOx in the flue gases. When used in conjunction with Low-Nox burners, the micronized coal allows a better control of the NOx emissions in the main combustion zone of the boiler and reduces substantially the unburned carbon carry-over with the fly ash. Micronized limestone, dolomite or hydrated lime are most valuable in the dry hot scrubbing of flue gases and affords a more effective aqueous scrubbing thereof as these particles have much larger reactive surfaces for the SOx and NOx scrubbing. This results in a more complete utilization of the sorbents resulting in substantial savings in the flue gas clean up section of a power plant in conforming to the requirements of the Clean Air Act.
The use of micronized coal and limestone/dolomite or lime as the fuel for generating electric power thus has significant advantages over the use of conventional fuels such as fuel oil or non-micronized coal and is much less expensive than natural gas.
In my U.S. Pat. No. 5,695,130 issued Dec. 9, 1997, a grinding system is described in which rotating screens with wide mesh openings are first used to comminute particulate material through spiral vortexes and such comminuted material is then fed to circular vortexes formed between rotating discs and stationary plates where the final grinding of the particulate material is accomplished and the final comminuted material is separated from the gas streams by centrifugal fans. The system and method of the present invention applies the basic technology of my prior patent in implementing the micronization of solid particles such as coal, limestone/dolomite, lime, and cement clinker. The system of the present invention, however, provides an improvement over the system of my prior patent by providing means to increase the production capacity of the micronizer through the use of a toroidal fluid bed generated with an outside pressurized gas and by minimizing the stress and erosion of the rotating elements and the vibration of the shafts and rotors employed thus lowering the system power requirements and lessening wear on the system. Further the device of the present invention employs a cascade of semi-permeable screens without the need of a centrifugal expelling fan downstream from each such semi-permeable screen as in the device of my prior patent. The device of the present invention, besides having vertical spiral vortexes generated by the rotating semi-permeable screens and accommodating the micronizing of the high speed particles, generates horizontal cyclones in the space above the rotating semi-permeable screens which achieves a centrifugal sorting of the comminuted particles whereby larger particles are recycled for further comminution. Thus, a final end product results without the need for using an outside classifier. In the device of the present invention, the use of a cascade of rotating semi-permeable screens provides for a graduated grinding of the ascending particles. This allows for control of the particle size and surface area in the final end product by varying the number of rotating semi-permeable screens in the vertical stack of such screens which forms the cascade.
The device and method of the present invention achieves the above indicated improvements by 1. utilizing a single chamber in which the shaft which drives all of the rotating elements is supported on a single pair of low friction bearings located at the top and bottom of the chamber; further, rather than employing a drive fan at the bottom of the chamber for driving the particles upwardly, a stationary velocity head is employed in conjunction with a pressurized air source to drive the particles upwardly along the walls of the chamber; 2. the rotation velocity for the rotating system components( rotors, and screens) is chosen such that the natural resonant frequencies of the system components are at least three times greater than this rotation velocity; 3. the rotors are designed so that they do not impact on the particles but rather generate kinetic energy in the form of centrifugal vortices in the air stream.
In the system of the invention, pressurized air is fed into the bottom of a chamber into which dry coarse particles to be comminuted are fed. Uplift action is provided by means of a stationary velocity head in the form of an inverted semi-ellipsoidal tank in conjunction with the pressurized air. The particles are first comminuted by a series of rotors designed to generate centrifugal vortices in the uplifting air which effect the comminuting action. The particles are then passed through a series of semi-permeable rotating screens which further effect the comminuting action. The fully comminuted particles are then driven out of the chamber to collecting chambers by means of fans or the gas stream.
A single chamber is employed to achieve the grinding action and all of the rotating elements in this chamber are mounted on and driven by a single shaft which rotates at a velocity of 2000-5000 rpm. The stress created in the rotating elements by the high centrifugal forces and the vibration of these elements and that of the shaft are overcome by design factors which are pointed out below.
It is therefore an object of this invention to provide a system and method for comminuting particles which consumes less power and results in less wear on the rotating and fixed elements employed.
It is a further object of this invention to provide a system and method which can be readily scaled up in production capacity and to enable the creation of a controllable particle size and surface area by varying the number of semi-permeable screens in a cascade thereof.
It is still a further object of this invention to provide a system and method for comminuting particles in which vibration of the system components is minimized.
Other objects of the invention will become apparent in view of the following description taken in connection with the accompanying drawings.